Three pairs of adjacent populations of Arrhenatherum elatius were studied both for their genetic diversity and for their genotypic structure. Each pair consisted of one population on a normal soil type and the other on spoil from mining. Using morphological characters as well as allozyme markers, the genetic diversity in populations on toxic soils (mining spoil) was found to be higher than in populations in normal pasture. This suggests that the tolerant populations have been built up from a large number of tolerant genotypes emanating from normal pasture populations in which tolerance genes are not uncommon. The higher genetic diversity of the tolerant population has then been maintained by the spatial heterogeneity of toxic habitats, or less intense selective forces of biotic origin (e.g. intraspecific competition) or by a more open breeding system. Variations in allozymes gave interesting clues. No clear cut differences in the genetic structure between pairs of populations were found, but there was a higher heterozygote deficit in denser populations. This might arise from inbreeding because of a more limited gene flow in dense populations. The higher genetic diversity of sparse populations on toxic soil might partly result from the effectively greater gene flow into these populations in which individuals must trap pollen from longer distances than do individuals in dense populations.